1. Field of Invention
This invention relates to systems and methods for generating binary irrational halftone dots.
2. Description of Related Art
When creating image regions using halftoning, binary clustered halftone dots are desirable. In particular, binary clustered halftone dots often produce the least amount of noise and the best highlights. Conventional halftoning adds a two-dimensional, spatially periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of essentially identical screens, are used to halftone each of the color image separation layers of a polychromatic, i.e., color, image. The halftone screens are oriented at different angles for printing the respective halftone color image separation layers.
Conventional halftoning methods, such as those disclosed in U.S. Pat. No. 5,410,414 to Curry, incorporated herein by reference in its entirety, warp, i.e., adjust or move, the image data produced by an image data generator to improve image characteristics, including registration. Such image data generators include gray scale image generators and binary image generators. However, merely warping the image data to improve image characteristics, including registration, results in offsets with the image data which have no corresponding offsets or warp in the halftone screens used to render color image separation layers.
Improving registration characteristics in halftone images conventionally includes also warping one or more of the halftone screens in a halftone screen system to correspond to the warping of the image data. This is disclosed in greater detail in U.S. Pat. No. 5,732,162 to Curry, incorporated herein by reference in its entirety. The 162 patent provides a detailed discussion of warping both image data and halftone screens.
A technique for reducing moire in halftone images includes generating binary clustered irrational halftone dots, for example, by adjusting halftone dot cluster size, is disclosed in U.S. Pat. No. 6,798,541, which is incorporated herein by reference in its entirety. The technique disclosed in the 541 Patent achieves reduced moire halftone images.
One common stimulus used by various halftone image forming apparatus to form images is a light beam scanned by a raster output scanner (ROS). A raster output scanner scans one or more such light beams across a photoreceptor drum or belt. In general, the raster output scanner scans each of the light beams across the photoreceptor drum or belt in a fast scan direction, while the photoreceptor drum or belt simultaneously moves relative to the scanned light beam in a slow scan direction.
As the one or more light beams are scanned across the photoreceptor drum or belt in the fast scan direction, the one or more light beams are individually modulated between off and on at a high rate. In particular, in various known high addressability systems, each light beam is modulated at a rate that is some integer multiple of, such as, for example, four or eight times, the period it takes the raster output scanner to move the one or more light beams a distance along the fast scan direction that is equal to the diameter of the light beams. This is known as 4× high addressability. As described in U.S. Pat. No. 6,208,430, which is incorporated herein by reference in its entirety, 4× or 8× high addressability allows the location at which the one or more light beams are turned on Thus, as a result, edges of image structures that are substantially perpendicular to the fast scan direction can be spatially controlled to one-quarter or one-eighth, respectively, of the diameter of the light beam along the fast scan direction.
However, for the process direction, the center-to-center spacing of two adjacent light beams or of two adjacent scans of a single light beam are offset by the diameter of the one or more light beams. That is, such scans, and thus of edges of image structures that are substantially parallel to the fast scan direction , cannot be offset by some integer fraction, such as ¼ or ⅛, of the diameter of the light beam in the fast scan direction. Therefore, when the edges of an image structure, such as a halftonedot, extend in directions that are substantially aligned with the fast scan direction, the light beam cannot merely be turned on when the current scan of the light beam intersects with the almost collinear image structure, such as a halftone dot, and left on until the light beam no longer intersects the image structure. Doing so would result in a significant error in the toner being applied to the resulting developed image at that area. This would result in that portion of the image having an image density that significantly departs from the desired image density represented by the image structure, such as the halftone dot.
Conventionally, intensity modulation is used to avoid this error in image density. In intensity modulation, the edge of the image structure, such as the halftone dot, extends along the fast scan direction, can be “dithered”, i.e., modulated, at a very high rate, so that the actual amount of image density of the developed image more closely corresponds to the image density of the overall image structure, such as the halftone dot.